Bhagavad-gītā, in the Thirteenth Chapter, you will find it is described: the body and the knower of the body. Just like you think over your body, you think over your finger, you will understand that it is your finger. When I think of this finger, I know this is my finger. When I think of this leg, I think that, "This is my leg." But I don't think your finger as my finger. This knower is individual, and he knows not everything but something of his body.
I do not know everything of my body. Suppose I am eating, I am eating something. How this eatable substance transforms into vitamin secretion and how it is being distributed all over the body and is supplying the energy? Or take, for example, I have got my hair, but I do not know how many hairs I have got. Is not that a fact? Can you count your hairs, how many hairs you have got? So, so many things we do not know even of our body, although I am claiming that "This is my body."
But there is another living being. He is supreme living being, Kṛṣṇa. He says that, "I know everything of everyone's body." Ksetra-jñaṁ cāpi māṁ viddhi sarva-kṣetreṣu bhārata (BG 13.3). This is the verse. Ksetra-jña means the knower of the body. You are individual knower of your body, I am knower individually of my body. So everyone is knower of his own body. But there is another person, who says that, "I know everything of everyone's body." Kṣetra-jñaṁ cāpi māṁ viddhi sarva-kṣetreṣu bhārata. He is known as Paramātmā.
The individual knower is known as ātmā, or soul, spirit soul.